1. Field of the Invention
This invention relates to a personal alarm system which comprises a portable transmitter unit to be worn about the person, or hand carried, and which is operable in an emergency to transmit an alarm signal which is to be received and processed by a central receiving station so that appropriate action can be initiated in response thereto.
2. Description of the Prior Art
There are many situations in which it is necessary, or advisable, for a portable transmitter unit to be available for use in emergencies, such as by the occupants of sheltered housing schemes, a warden on routine visits to such occupants, or medical personnel in hospitals or other institutions. In these situations, it is important, when an emergency call is received at the central control or command station, that the location from which the emergency call has been made is immediately discernable, and therefore it is usual to locate a dedicated receiver at each location to be monitored e.g. on the wall or ceiling of a room, and for each dedicated receiver to be connectable in any desired manner e.g. radiowave communication or direct electric line connection to the central control station, where the re-transmitted emergency signal will be indicated at the control station as having come from that particular location.
It is known to use ultrasonic personal alarm transmitter units, which issue ultrasonic signals to be received by a dedicated ultrasonic receiver at each location which is to be monitored (which then re-transmits to the central station), but ultrasonic units rely upon crystal devices, which are not robust, and in fact are rather fragile, so that this can cause problems with regard to reliability. In addition, it is a feature of existing ultrasonic transmitters that they cannot readily be tested as to their current state of serviceability while in use, and there is therefore a risk that emergency signals may fail to be issued and/or received.
It is also known to provide each member of staff in a hospital or other people-care type institution with his own radio transmitter unit, for use in emergencies and which transmits a radio signal which is picked-up by a central receiver, and this gives an indication of the caller, but not of the location where the caller has made the emergency call. Therefore, the radio transmitter units are not suitable for use in situations in which the users may be located in any one of a number of different locations when the emergency call has been made.